Sera from outpatients attending county health department clinics in areas of California with consistent (Imperial Valley) and sporadic (Sacramento Valley) enzootic transmission of western equine encephalomyelitis (WEE) and St.

Seroprevalence for SLE virus in Imperial County increased as a function of both age and years of residence, indicating that this virus was endemic with a low rate of annual infection.

Of 26 sera that tested positive for SLE virus antibody by an enzyme immunoassay, but were negative by plaque reduction neutralization test, 14 (53%) had neutralizing antibody that reacted with = one type of dengue (DEN) virus.

The low prevalence of neutralizing antibody for WEE and SLE in the California human population indicated that despite recent increases in enzootic transmission, contact between humans and infectious mosquitoes have remained low.